1. what is profibus ? 2. what is modbus?

1. PROFIBUS (Process Field Bus) is a standard for fieldbus communication in automation technology and was first promoted in1989 by BMBF (German department of education and research) and then used by Siemens. PROFIBUS can reduce operating costs, increase productivity, decrease time to market for new products and improve product quality. And, unlike standard4-20mA controls, PROFIBUS can support up to32 devices per segment - up to a total of126 devices, depending on total system current.

Using shielded twisted pair cable, the PROFIBUS topologies utilized in the industrial environment include the following configurations: line, tree and star (and combinations of these topologies). The network can support up to126 nodes by using repeaters, but the number of repeaters is limited to nine. Standard connections are made through use of a9-pin D-Subminiature connector. Transmission speeds are selectable from9.6kbps to12Mbps.

There are variations of PROFIBUS in use today:

•PROFIBUS DP (Decentralized Peripherals) is based on RS485 transmission technology, used to operate sensors and actuators via a centralized controller in production (factory) automation applications. For field devices which do not draw the required power from the bus, the standard copper-based RS485 interface can be implemented. This provides greater flexibility when using the field device, because it can be connected to PROFIBUS DP without a segment coupler or link. Data rates from9.6 Kbps to12 Mbps are supported. RS485 modules are available from various manufacturers including an intrinsically safe version RS485-IS.

•PROFIBUS PA (Process Automation) is used to monitor measuring equipment via a process control system in process automation applications. This variant is designed for use in explosion/hazardous areas (Ex-zone0 and1). The Physical Layer (i.e. the cable) conforms to IEC61158-2, which is characterized by the term “Manchester Coded and Bus Powered” that allows power to be delivered over the bus to field instruments and provides a typical supply current of10 -15 mA on the bus cable, while limiting current flows so that explosive conditions are not created, even if a malfunction occurs. The number of devices attached to a PA segment is limited by this feature. PA has a data transmission rate of31.25 kbit/s. However, PA uses the same protocol as DP, and can be linked to a DP network using a coupler device. The much faster DP acts as a backbone network for transmitting process signals to the controller. This means that DP and PA can work tightly together, especially in hybrid applications where process and factory automation networks operate side by side.

•PROFISafe is an integrated safety technology for use in all sectors of discrete manufacturing and process automation. It is independent of the communication method and provides cost-effective and flexible functional safety

•PROFIDrive is a supplement to the PROFIBUS standard, a unified device behavior and access technique to the drive data. For the user, this means that engineering costs are reduced when planning and implementing plants and systems, as the various drives respond the same way to control instructions. The programming costs are significantly reduced by using standardized program blocks in the open-loop and closed-loop control systems for controlling the drive units via PROFIBUS.

2. Modbus is a serial communication protocol developed by Modicon published by Modicon in1979 for use with its programmable logic controllers (PLCs). In simple terms, it is a method used for transmitting information over serial lines between electronic devices. The device requesting the information is called the Modbus Master and the devices supplying information are Modbus Slaves. In a standard Modbus network, there is one Master and up to247 Slaves, each with a unique Slave Address from1 to247. The Master can also write information to the Slaves. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Versions of the Modbus protocol exist for serial lines (Modbus RTU and Modbus ASCII) and for Ethernet (Modbus TCP). Information is stored in the Slave device in four different entities.

Two entities store on/off discrete values (Coil Status and Input Status) and other two store numerical values (Input register & Holding Register). The coils and registers each have read-only type (Input Status & Input Register) and read-write type (Coil Status & Holding Register).

PROFIBUS (Process Field Bus) is a standard for fieldbus communication in automation technology used by Siemens.

Modbus is a serial communications protocol originally published by Modicon (now Schneider Electric) in1979 for use with its programmable logic controllers (PLCs)